Scaling up global kinetic models of pulsar magnetospheres using a hybrid force-free-PIC numerical approach

The particle-in-cell approach has proven effective at modeling neutron star and black hole magnetospheres from first principles, but global simulations are plagued with an unrealistically small separation between the scales where microphysics operates and the system-size scales due to limited numerical resources. A legitimate concern is whether the scale separation currently achieved is large enough, such that results can be safely extrapolated to realistic scales. In this work, our aim is to explore the effect of scaling physical parameters up, and to check whether salient features uncovered by pure kinetic models at smaller scales are still valid, with a special emphasis on particle acceleration and high-energy radiation emitted beyond the light cylinder. To reach this objective, we develop a new hybrid numerical scheme coupling the ideal force-free and the particle-in-cell methods, to optimize the numerical cost of global models. We propose a domain decomposition of the magnetosphere based on the magnetic field topology using the flux function. The force-free model is enforced along open field lines while the particle-in-cell model is restricted to the reconnecting field line region. As a proof of concept, this new hybrid model is applied to simulate a weak millisecond pulsar magnetosphere with realistic scales using high-resolution axisymmetric simulations. Magnetospheric features reported by previous kinetic models are recovered, and strong synchrotron radiation above 100MeV consistent with the Fermi-LAT gamma-ray pulsar population is successfully reproduced. This work further consolidates the shining reconnecting current sheet scenario as the origin of the gamma-ray emission in pulsars, as well as firmly establishes pulsar magnetospheres as at least TeV particle accelerators.